Many conventional receivers, when operating in “idle” mode, “wake up” periodically to determine if any messages (pages) are scheduled to be transmitted to the receiver. If no messages are scheduled, the receiver will power down in order to extend the battery life of the receiver. One such communication system currently being developed with such power-saving capabilities is the next generation Code-Division Multiple-Access (CDMA) cellular communication system, more commonly referred to as cdma2000, or IS-2000. As illustrated in FIG. 1, cdma2000 utilizes a plurality of 20 millisecond (ms) synchronous frames 102 (shown as F0, F1, F2, . . . , FK). Frames 102 are transmitted during a periodically occurring time span corresponding to a transmission cycle which has a predetermined duration (e.g., 16* 0.080*2N seconds, where N is zero or a positive integer). These frames are grouped together into a slot containing four frames. A mobile station within a cdma2000 system is assigned a particular slot in which all unsolicited messages for the particular mobile station are to be transmitted. A mobile station operating as such is said to be operating in a “slotted mode.” Slotted mode operation allows a cdma2000 mobile station to power up for a single assigned paging slot every 1.28*2N seconds.
In order to further conserve power, all addresses for mobile stations that are to receive messages during a particular slot are broadcast prior to broadcasting page data. If a mobile station's address is not broadcast, the mobile station can power down for the remainder of the slot. FIG. 2 shows slot 200 having four frames. As shown, a first portion 201 of slot 200 contains address information for all mobile stations that have page data within slot 200. A particular mobile station assigned to slot 200 will awake during the transmission time for slot 200. The mobile station will receive the first frame, and if the mobile station's address is not contained within the first portion 201 of slot 200, the mobile station will power down prior to receiving the rest of slot 200. After a period of time, the mobile will power up again and repeat the process.
Recently it has been proposed to add dispatch capabilities to the cdma2000 system. Unlike the interconnect services provided by today's cellular systems, dispatch services have been traditionally provided by two-way radio systems. Such services allow a user to communicate in ways that are difficult or costly using today's cellular systems. The dispatch group call service, for example, enables a user to communicate with a group of people simultaneously and instantaneously, usually just by depressing a push-to-talk (PTT) button. Using a cellular system, such a call could not occur instantaneously since either telephone numbers would need to be dialed for a three-way call or arrangements would need to be made to setup a conference call.
Likewise, the dispatch individual call service enables a user to communicate with another user quickly and spontaneously. This feature is ideal for two people who are working together but are unable to speak with one another directly such as two people working in concert but in different parts of a building. Where a wireless telephone call is more appropriate for a conversation, short messages between two people as they work are better facilitated by the dispatch individual call service.
A problem encountered when dispatch capabilities are added to a system capable of slotted mode operation is that the minimum slot cycle duration typically used for standard calls is unacceptably long for those remote units operating in dispatch mode. More particularly, because the minimum slot cycle duration is 1.28 seconds, there is on average a 640 ms delay when paging the mobile. Although this 640 ms delay is acceptable for a typical interconnect voice call, it is not acceptable for dispatch services which rely on a very fast connection being made to the called party.
Because of this, a need exists for a method and apparatus for assigning a slot cycle that reduces the time it takes to make a dispatch call.